Stock Car Bumpsteer - Get The Bump Out

When you drive over a bump or corner hard on the racetrack, does your car appear to have a mind of its own, with the front wheels veering one way or the other? When the front suspension responds to these bumps or body roll inputs from the track with toe-in or toe-out while moving through its bump (compression) and rebound travel, that's bumpsteer. It's undesirable because your control of the steering and traction is compromised. It's a problem that could be costing you valuable time on the track.

Ideally, bumpsteer can be avoided when a car is built. The key is to have a basic understanding of suspension and steering geometry, and to have the right parts and pieces when you start out.

"The most trouble local racers have is that when they get going, perhaps they don't have the right parts in hand," says Jeff Butcher, president of Longacre Racing Products. "Sometimes when you don't have the right parts for a specific car, there's really no way to get the bump right. It's something that has to be designed in. A local guy might have to buy something that he's not absolutely sure about bolting on. You have to be careful there."

If you're buying a rolling chassis, check with the builder to learn his normal setup for bumpsteer. This could save time and energy as you get to know the car and what it will do. If you have a new chassis, check bumpsteer early on, then check it periodically after the car has some laps on it.

Bumpsteer can make a car unpredictable and hard to control, especially on older paved tracks that may be worn and bumpy. If you're using a bumpsteer gauge and the plate moves toward the engine, you have a bump-in condition; if the plate moves away from the engine, you have bump-out.

The goal is to have zero bumpsteer or to have as little as necessary for a well-handling car. Butcher recommends this method of accomplishing zero bump: The tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper A-arm pivot and the lower control-arm pivot. In addition, the centerline of the tie rod must intersect with the "instant center" created by the upper A-arm and the lower control arm (see Figure 1).

Instant center, according to Butcher, is an imaginary point created by drawing a line from the upper A-arm ball joint through the A-arm pivot, where it is intersected by an imaginary line that extends from the lower ball joint through the inner control-arm pivot. The two imaginary lines intersect at instant center.

To achieve zero bump, Butcher says the tie rod must travel on the same arc as the suspension when the car goes through travel. It involves matching lengths and arcs to prevent any unwanted steering of the front tires.

Butcher says that some bump-out can make the car more stable on corner entry, but bump-in is, as a rule, undesirable. Regardless of how much bumpsteer is built into a car, it's a concept that must be recognized and respected.

"Every tenth of a second is going to make a difference on the track," says Kevin Smith, vice president of Irvan-Smith Inc. "If it's a tenth of a second or a half a tenth a second, in a 50-lap race or a 100-lap race-if you're making that much better time than the next guy every lap-consider how much farther ahead you're going to be at the end of the race. Setting bumpsteer is fine-tuning the race car, and it's just as important as a half-pound of air pressure, trailing arm angles, offset rearends, a sixteenth of an inch here or there, stagger, or anything else. This is just another piece of the puzzle that gets you one step closer to being complete."